When applying radiation to the lesion of a radiotherapy target subject, an irradiation field limiting device has been widely used which forms an irradiation field corresponding to the shape of the lesion by limiting the irradiation range.
Japanese (unexamined) Patent Application, No. H06-300896 (hereinafter called “patent document 1”) discloses an irradiation field limiting device which includes an aperture block (hereinafter called “aperture leaf”) of which the surface of the aperture operation which limits the irradiation range forms a cylindrical surface and which is provided with a rack in the inner circumferential surface, and a pinion which engages the aperture leaf, in which the aperture leaf is driven by transmitting rotation of a motor to the rack and the pinion through a chain or the like.
Japanese Examined Patent Application, No. H07-10282 (hereinafter called “patent document 2”) discloses an irradiation field limiting device which includes a plurality of aperture leaves in which grooves and protrusions are formed on the sliding surface, in which the aperture leaves are slidably arranged in the side surface direction to shield radiation passing through the opening between the sliding surfaces.
The irradiation field limiting device disclosed in the patent document 1 has a problem in which the positional accuracy of the aperture leaf decreases due to wear of the rack and the pinion. Therefore, it is difficult to accurately form the irradiation field.
The widths of the rack and the pinion cannot be reduced to a large extent taking durability and the like into consideration (for example, durability significantly decreases when reducing the width to 2 mm or less). This makes it difficult to form an aperture leaf with a reduced thickness. As a result, a large number of aperture leaves cannot be disposed in a limited installation space.
When the shape of the aperture leaf is linear, the aperture leaf operates linearly. Therefore, the aperture leaf can be driven with high accuracy in comparison with the irradiation field limiting device disclosed in the patent document 1 which uses the rack and the pinion by forming a female thread portion in the aperture leaf and forming a male thread portion on a drive shaft connected to a motor.
In this case, when reducing the thickness of the aperture leaf, the diameters of the female thread portion and the male thread portion must be correspondingly reduced. As a result, the female thread portion and the male thread portion exhibit reduced ridge strength. Therefore, it is difficult to ensure stable operation over a long period of time since the ridges easily wear and exhibit poor lubricity.
The irradiation field limiting device disclosed in the patent document 2 has a problem in which it is difficult to form a thin aperture leaf because the groove is formed in the sliding surface of the aperture leaf.